936 



EOCENE AND OLIGOCENE TITANOTHERES OF MONGOLIA 



extending to the base, facing internally, a similarity 

 due to relationship to or convergence with Menodus. 

 The marked incisive distinction from Protitanothe- 

 rium is that the lateral incisors (If) are smaller than 

 the second incisors (If) and even somewhat smaller 

 than the median incisors (l\). All the incisors are 

 very prominent, flattened anteroposteriorly, not 

 deeply cupped behind, gently postcingulate. 



High laterally compressed inferior premolar series 

 with highly sculptured crowns, a flattened pi, a highly 

 flattened p2 with shallow talonid basin, a somewhat 

 broader ps with broad talonid basin, a still broader 

 P4 with talonid basin broader than that of the trigonid. 

 These characters clearly distinguish the premolar 

 teeth from those of both DolicJiorJiinus and Pro- 

 titanoiherium—m fact, the lower premolars are wholly 

 different. The same is true of the laterally compressed 

 superior premolars with prominent sculptured ecto- 

 lophs, laterally compressed deuterocones with very 

 feebly indicated rudiments of tetartocone ridges and 

 without distinct tetartocone prominence. 



Harmonic with these laterally compressed premolars 

 are the extremely elongate, laterally compressed molar 

 teeth, the narrowest teeth of the kind to be seen 

 among the titanotheres, distinguished by two deep 

 concavities between the ectoloph and the inner por- 

 tion of the crown, these depressions reminding us of 

 those developed in a few of the Oligocene titanotheres. 

 On the inner side of these narrow crowns lie fairly 

 prominent protocones and hypocones. In m^ there is 

 a trace of a protoconule and the narrow postero- 

 internal shelf of the crown bears a very prominent 

 hypocone, a relatively rare feature in titanotheres and 

 very distinctive as compared with the absence of a 

 hypocone in m' in both Protitanoiherium and DolicJio- 

 rhinus. 



The above dental characters are displayed in the 

 second paratype specimen (Am. Mus. 20121) belonging 

 to a more aged individual; also in a more progressive 

 individual wherein we observe in the superior pre- 

 molars tetartocones increasing in prominence and in 

 distinctness as we pass from p^ to p*, a progressive 

 featm'e of premolar development not observed in 

 either of the other Irdin Manha species. A third 

 maxiUa (Am. Mus. 20106), associated with the type 

 jaw, displays a more rudimentary condition of the 

 tetartocones in the third superior premolars. 



Consequently among the materials referable to this 

 genus and species we observe several stages of pro- 

 gressive development in the evolution of the superior 

 and inferior premolar teeth. 



SUBFAMILY BEOKTOPINAE 

 Brontops Marsh, 1887 



Generic characters. — The generic characters of this 

 broad-headed Mongolian titanothere are not readily 

 derived from the badly crushed skull, symphysis of 



lower jaw, and single lower molar found in the Ardyn 

 Obo formation, of lower Oligocene age. The cranial 

 proportions are short and broad; the robust, widely 

 arched zygomatic arches are correlated with strong 

 powers of mastication; the breadth-length index of 

 74, estimated from the reconstructed skuU (length 

 576 mm., breadth 424 mm. — actual 500 mm.), con- 

 trasts with the index of 58-69 in the widely arched 

 Protitanotherium and of 47 in the narrowly arched 

 DolicJiorJiinus. The cranium is actually much shorter 

 than that of either DolicJiorJiinus (660-695 mm.) or 

 ProtitanotJierium (695-757 mm.). The horns partake 

 of the short, broad character of the cranium; they 

 are obtusely prominent. The elongated nasals are 

 broad and shovel-shaped, as in ProtitanotJierium and 

 TitanotJierium. The symphysis of a referred jaw is 

 extremely massive, like that of Brontops. The dental 

 formula, Iflf, C{:\, P|:|, M|:|, agrees with that of 

 Teleodus Marsh in the presence of three superior 

 incisors and differs from that of Brontops IracJiy- 

 cepJialus Osborn, in which the incisive formula is 

 Iflf. The cranium is as br achy cephalic as that of 

 Brontops bracJiycepJialus, as shown in the full descrip- 

 tion of B. hracJiycepJialus above. The strong, well- 

 developed tetartocones of the premolars or double 

 internal premolar cones mark the chief progression 

 beyond ProtitanotJierium. Most of the measurements 

 in the following table are estimates, as the skulls 

 were crushed. 



Measurements of Brontops gobiensis and B. brachycephalus, in 

 millimeters 



Basilar length of skull (anteroposterior) 



Transverse zygomatic width 



Breadth-length or cranial index 



P2-m3 --- 



M^, anteroposterior 



M^, transverse 



M', index 



B. gobien- 

 sis, Am. 

 Mus. 20354 

 (type) 



"■576 

 "424 

 °74 

 300 

 72-78 

 64-70 



B. brachy- 

 cephalus, 

 Am. Mus. 

 1495 (re- 

 ferred) 



570 

 337 



286-291 



61 



68-70 



111-115 



Brontops gobiensis Osborn, 1925 



Original reference. — Osborn, H. F., Upper Eocene 

 and lower Oligocene titanotheres of Mongolia: Am. 

 Mus. Novitates No. 202, 1925. 



Type description (Osborn, 1925, p. 5). — "In the 

 Ardyn Obo formation were found remains of three 

 titanotheres in a distinctly OHgocene stage of evolu- 

 tion. The type skull (Am. Mus. 20354), named 

 herewith Brontops gohiensis, contains three superior 

 incisors with distinct tetartocones in the three molars; 

 no hypocone in m^ The massive symphysis (Am. 

 Mus. 20353) is referred to the same species." 



